In the past, avionics display engineers have attempted to enhance the display of information to pilots and still retain many of the basic configurations and appearances of earlier displays. Much effort has been expended in this pursuit. However, frequently the "improvements", which are designed to enhance the pilot's efficiencies, are more confusing, less attractive and more difficult to master than the displays which they are purported to improve. One particular area that has been proven to be especially difficult is the use of PFDs having vertically oriented linear altimeters therein. The vertical scale linear altimeter design is attractive, as a general concept, because it allows for easy orientation of the primary flight instruments on a single display device, while preserving the classic T-shaped configuration of the airspeed indicator, attitude indicator, course indicator, and altimeter combination.
One attempt, as is shown in FIG. 1, has been to utilize two moving numbered tapes, where one tape represents a fine scale, while the other tape provides a coarse scale. This approach has several serious drawbacks; with a predominant problem being the increased likelihood of pilot confusion caused by the presence of several four or five digit numerals displayed at the same time, only one of which is relevant.
Another PFD, as is shown in FIG. 2, has been proposed where a single coarse tape is used together with a fine scale having a rotating disk for least significant digits, but it also, has undesirable features. Most importantly, both the fine scale and the coarse scale have a relatively small range of display altitudes. Typically, a coarse scale of limited range makes level-offs from climbs and descents more difficult. Similarly, a fine scale with a small range of altitudes often makes it difficult for the pilot to hold altitude without constantly looking directly at the display symbology.
Consequently, there exists a need for further improvement in PFDs which facilitate the display of important information to the pilot with no concomitant increase in pilot confusion caused by flight instrument complexities.